Having spent numerous years recruiting sales people, both as a recruitment consultant and as an employer, I have found one thing a consistent:
Treat people poorly or be too gready and they will leave.
Good salespeople will stay in a role if they feel they are being treated well and are appreciated.
For example: If a comm only sales person brings ion a new client and then you reduce the commission for all future business form that client, what are you saying? I appreciate you generating lots of profit for me, but once you have done the hard bit, I want all the reward! He won't stay long.
For comm only, you can expect a high churn rate. However, if there is a solid opportunity to earn a realistic living and it is being achieved , there is no reason for the individual to leave, especially if you continue to pay commission on all future orders. If the individual has the opportunity to develop a good pipeline with solid revenue streams, they are more likely to stay for the long term earnings, and therfore earn you more money.
As to how much commission to pay, this one is simple. How much additional business/profit is the individual generating for you that you would not previosly have received. So, as long as you are making money, make sure you look after them - pay well. What is wrong with a 70/30 split on profit in their favour, they won't leave if they are earning good money and know the conditions are good. The 30 is pure profit to you that you would not have otherwise had.
The dnager when recruiting is to be too clever or drive to hard a bargain, it will only come back and bite you in the ass later. If your sales people leave, chances are, so will the clients they brought in - people buy from people!